Aqueous Partition Mechanism of Organophosphorus Extractants in

Article pubs.acs.org/IECR

Aqueous Partition Mechanism of Organophosphorus Extractants in Rare Earths Extraction Wenrou Su,†,‡ Ji Chen,*,† and Yu Jing† †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, P. R. China ‡ University of Chinese Academy of Sciences, Beijing, 100039, P. R. China ABSTRACT: The aqueous partition mechanism of traditional extractants and ionic liquid for the extraction of rare earths (REs) is explored in this work. To investigate the aqueous partition of extractants, the aqueous solubility of di(2-ethylhexyl) phosphoric acid (P204), 2-ethylhexyl phosphoric acid mono-2-ethylhexyl ester (P507), di(2-ethylhexyl) phosphinic acid (P227), bis (2,4,4-trimethylpentyl) phosphinic acid (C272), and [trialkylmethylammonium][di(2-ethylhexyl)orthophosphinate] ([A336][P507]) was analyzed systemically. The results demonstrated that the solubility of extractants decreased with increase of aqueous acidity, RE loading, and electrolyte concentration. Especially, the solubility of P204, P507, P227, and C272 decreased with the increase of RE complex, indicating that aqueous partition of the extractants was accompanied by the RE coordination reaction. The electrical double layer theory and the Pitzer equation was used to explain the inhibition of electrolyte on the aqueous partition of extractants. In addition, a solubility of [A336][P507] lower than that of saponified P507 illustrated that the partition behavior was related to extractant property.

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and used in RE separation in our laboratory.18−20 The structural formulas of these compounds are illustrated in Figure 1.

INTRODUCTION Solvent extraction that exploits the partition of a solute between two immiscible liquid phases accompanies the exchange and transport of substances. Solvent extraction has been extensively used in rare earths (REs) extraction and recycling, owing to its large operation capacity and good separation effect.1−3 The ultimate aim of RE extraction is to selectively transfer a lanthanide mixture from the aqueous phase into the oil phase, based on the different solubilities of the metal−extractant complex in two phases.4−6 The amphiphilic extractant constituted by a polar complexation part and a hydrophobic part plays the role of the solubilization agent, transferring the metal ions in the hydrated state from water into oil and in the solvation state solubilized in reverse micelle.7,8 Therefore, investigating the physicochemical property of the extractant is important for understanding the extraction mechanisms of RE. Organophosphorus acid extractants are widely employed in RE extraction, for example, di(2-ethylhexyl) phosphoric acid (P204) and 2-ethylhexyl phosphoric acid mono 2-ethylhexyl ester (P507) exhibit a remarkable selectivity on chemically similar REs.9−11 Furthermore, di(2-ethylhexyl) phosphinic acid (P227) and bis (2,4,4-trimethylpentyl) phosphinic acid (C272) are generally considered as alternatives in RE extraction.12−15 In particular, in China, P507 is the main industrial extractant applied to the extraction and separation of RE. In recent years, researchers put forward some novel extraction systems to improve the extraction process.16,17 As one of these systems, bifunctional ionic liquid extractants (Bif-ILEs) [A336][P507] have been synthesized by the acid/base neutralization method © XXXX American Chemical Society

Figure 1. Structural formulas of P204(A), P507(B), P227(C), C272(D), and [A336][P507](E).

Organophosphorus acid extractants typically consist of the hydrophobic part alkyl chains and the hydrophilic grouping of P(O)OH. Furthermore, they exhibit good interfacial activity21 and form a film which could absorb metals ions with poor solubility in oil. In the acidic organophosphorus extraction, the metal−extractant complex not only showed different aggregation behaviors but also formed association microstructures.22 Recent research focused on extractant properties in the organic Received: May 3, 2016 Revised: July 13, 2016 Accepted: July 14, 2016

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DOI: 10.1021/acs.iecr.6b01709 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research phase.23,24 However, the distribution of amphiphilic extractants between two phases could not be negligible in the extraction process. RE extraction includes dissolving, partition, and dissociation of the extractants.25 The oil/water partition of extractant had an influence on the mass transfer.26 Kolarik27 reported some methods about the calculation of an extractant’s partition coefficients. These fundamental studies reflected a general rule of the extractant’s partition in two phases. Especially, the aqueous partition is closely related to the aqueous solubility of the extractant. The change of the aqueous solubility of extractant could adjust the oil/water interface arrangement.28 In addition, the dissolution of organophosphorus acid extractant led to a loss in the acid medium and metalsalts feed.29 In some conditions, extractant dissolved and formed aggregates in the aqueous phase.30 Wang et al.31 found that the organophosphorus acid extractants formed aggregates in the equilibrated aqueous phase. Although the aqueous partition of the extractant has an effect on the extraction process, there was little information about the aqueous solubility of the extractant in the actual extraction. The calculation of the extractant’s partition might not be enough for the prediction of the specific values.27 Moreover, Wang et al.31 showed that the loss of a low saponification percentage of P204 is slight in an equilibrated aqueous phase and could not be determined by Fourier transform infrared (FT-IR) analysis. Besides, there was little data about the aqueous solubility of extractants in the raffinate of the RE extraction. Therefore, the aqueous partition of extractant needs to be evaluated quantitatively, and the influencing factors of the extractant’s partition need to be discussed. Electrolytes have an effect on the aqueous partition of the extractants. By adding strong electrolytes into the aqueous phase, the dissolution of organophosphorus acid extractants can be inhibited.30 The Pitzer equation is usually used to calculate the activity coefficients of the components in the equilibrated aqueous phase.32,33 For the most part, these investigations are confined to calculating the activity coefficients of an electrolyte in the simple aqueous phase, but a few comparable studies are available regarding the activity coefficient of extractant in the RE extraction. Therefore, it is necessary to use the Pitzer equation to describe the effect of electrolytes on activity coefficients of the extractant in a RE extraction. The exploration of a new system is also equally important. Ionic liquids, a new medium of RE extraction, have the potential to develop a sustainable process of RE separation. But industrial applications of ILs also face some challenges, especially, in the process of extraction when the cations and anions of ILs or the ionic complex are dissolved in the aqueous phase.34,35 Freire et al.36 reported that the solubility of ILs was influenced by the alkyl chain length and temperature. But the aqueous solubility of ILs in the actual extraction is rarely mentioned.37 Therefore, we need to know the aqueous partition of [A336][P507] for the sake of industrial application. Furthermore, the difference of the aqueous partition between P507 and [A336][P507], which used the tricaprylmethylammonium ion ([A336]+) as a substitute for the conventional cations (H+ or NH4+) of P507, needs to be clarified. In this paper, we present the quantitative analysis of organophosphorus acid extractants (P204, P507, P227 and C272) and the Bif-ILs ([A336][P507]) distributed in the aqueous phase by measuring the solubility of the extractants in a RE extraction. This study also includes the existing form of extractants in the aqueous phase and the effect of aqueous

acidity and the electrolytes on the solubility. In addition, the effect of cations on the aqueous partition of extractants is demonstrated by the comparison of saponified P507 and [A336][P507].

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EXPERIMENTAL SECTION Reagents. P204 (>95%) and P507 (>95%) was purchased from LuoyangAoda Chemical Co., Ltd. P227 (>99%) was kindly supplied by Shanghai Institute of Organic Chemistry. C272 was kindly supplied by Cytec Canada, Inc. Tricaprylmethylammonium chloride (Aliquat 336) was purchased from ACROS. [A336][P507] was synthesized according to a published method.18 All extractants were used without further purification. All of the extractants were diluted with n-heptane. The aqueous acidity was adjusted by hydrochloric acid. Stock solutions of REs(III) were prepared via dissolving their oxides (>99.99%) in concentrated hydrochloric acid. All the other chemicals were analytical grade. All aqueous solutions were diluted with deionized water. Methods. To control the acidity of the aqueous phase, we used the saponified extractant. It was prepared by adding a desired amount of the concentrated ammonia to the extractant in n-heptane and by stirring the mixture for 48 h to form a single phase. The saponification percentage was determined by a titration method with NaOH solution. The saponification percentage of P204, P507, P227, and C272 was kept at 36 ± 1%, which is generally used in the RE industrial extraction, except in the acidity experiment. The solubilities of P204, P507, P227, C272, and [A336][P507] were measured at different aqueous acidity, different loading ratios (defined as the ratio of the concentration of the saponified extractant coordinating with REs to the total saponified extractant), and different electrolytes. The experiments were performed by the equal volume of organic and aqueous solutions in equilibrium tubes and for 30 min with a constant temperature bath at 25 °C. The mixture was then centrifuged for 5 min at 3000 rev min−1 to enhance phase separation. The solubility of extractants in the aqueous phase was analyzed as phosphorus concentration by inductively coupled plasma optical emission spectrometers (ICP-OES, Thermo iCAP 6000). The existing form of P204, P507, P227, and C272 in the aqueous phase was determined by MALDITOF-MS (Waters, Quattro Premier XE). The existing form of [A336][P507] was identified by HPLC, equipped with a pump (515 HPLC Pump, Waters), a refractive index detector (Waters 2414), and a SunFire C18 analytical column. The column temperature was kept at 25 °C; the eluent was methanol. Flow rate was 1 mL/min. RE concentration was determined by EDTA titration using xylenol orange as indicator. The experiments were conducted in duplicate under the same conditions, and the relative error between the duplicates was less than 5%. The results were reported as mean values. Determination of the Existing Form of Organophosphorus Acid Extractants in Aqueous Phase. The electrospray ionization mass spectrometry (ESI-MS) method has been developed to determinate organic acid38 and its metal−ligand complexes.39 In this study, the organophosphorus extractants were detected by the [M−H]− mode. The existing form of extractant in the aqueous phase is determined by ESI− MS data (Figure 2a−d). As observed, the mass spectrometry signals showed plenty of ions at m/z 321.3, 305.3, 289.3, 289.1 corresponding to the versions of the extractants anion, i.e., [(RO)2P(O)O]−, B


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RESULTS AND DISCUSSION 1. Effect of Aqueous Acidity on the Solubility of Organophosphorus Acid Extractants. It is well-known that the aqueous acidity has an important influence on the extraction process.11,25 So we investigated the effect of aqueous acidity on the solubility of extractants. The results were shown in Figure 4. It was found that the solubility of the four

Figure 2. ESI-MS spectrum of extractants in aqueous phase: (a) P204, (b) P507, (c) P227, (d) C272.

Figure 4. Effects of aqueous acidity on the solubility of extractants. [P204], [P507], [P227], and [C272] = 0.03 mol L−1.

[(RO)RP(O)O]−, [R2P(O)O]−, and [T2P(O)O]− respectively, where R represents 2-ethylhexyl and T represents 2,4,4trimethylpentyl. The four extractants exhibited the same ionization behavior by losing a hydrogen proton. This result suggested that the dissolved extractant in the aqueous phase does not exist as a form of the metal−complex. Determination of the Existing Form of [A336][P507] in the Aqueous Phase. The existing form of [A336][P507] in the aqueous phase is determined by the HPLC method, which is a rapid and precise analytical technique used to separate and identify ILs.18,41 The results are shown in Figure 3.

extractants increased with the decrease of acid concentration. In the higher acidity range, hydrogen ions could suppress the dissociation of extractants and lead to the decrease of extractants distributed in the aqueous phase. With the decrease of acidity, the solubility of the four extractants in the aqueous phase was increased. When the pH reached to the acid dissociation constants (pKa) of the extractants, respectively42 (listed in Table 1), the solubility of the extractants was almost Table 1. Physicochemical Parameters of Extractant pKaa Vb (Å3)

P507

P204

P227

C272

3.30 429.65

2.79 421.30

4.98 403.13

3.73 391.56

a

The acid dissociation contants (pKa) of P204, P507, C272 are cited from ref 44, but the value of P227 was provided from Shanghai Institute of Organic Chemistry. bThe molecule volume (V).

unchanged, which showed the great effect of the dissociation of the acid extractants on the aqueous partition. The increased sequence of solubility was P227, C272, P204, and P507. The extractants dissolved in the aqueous phase will destroy the intermolecular forces to form a cavity to accommodate the extractant molecules. The cavity effect is proportional to the molecular volume of the extractants.25 Therefore the molecular volume of extractants was calculated and listed in Table 1. As presented, the larger molecular volume of extractants had a higher solubility. Nonetheless, the molecular volume of C272 was smaller than P227, while the solubility of C272 was bigger. Because the alkyl chains of C272 are more branched, it will affect the cavity volume of extractant in water.43 These results clearly indicated that the aqueous partition of extractants was affected not only by the acid dissociation constant but also by its structure. 2. Effects of REs on the Solubility of Organophosphorus Acid Extractants. To determine the influence

Figure 3. Liquid chromatogram of [A336][P507].

As present, the IL was eluted completely in 5 min. The part of [A336][P507] distributed in the aqueous phase and pure [A336][P507] had the same retention time, suggesting that the dissociation of [A336][P507] did not occur in the aqueous phase due to the inner synergistic effect.19 C


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It can be seen that the extractant HL is distributed in the aqueous phase and then the dissociated extractant L− will coordinated with RE ions. Because the preferred RE complex (MLn) is insoluble in the aqueous phase, it shifts quickly into the interface.40 Subsequently, MLn is carried by the acidic extractant monomer (HL) and then extracted into the organic phase.45 Therefore, the aqueous partition of organophosphorus acid extractants accompany the RE coordination reaction. 3. Effects of Different Acidities in Initial RE Solution on the Solubility of Organophosphorus Acid Extractants. The adjustment of aqueous acidity is often used in the process of the separation of REs, and the effect of acidity of initial RE solution was illustrated in Figure 6. As presented, the

of REs on the solubility of extractants, three REs, La, Gd, and Yb were used as representative. Figure 5 listed the solubility of the four extractants in different loading ratios.

Figure 5. Effects of REs loading on the solubility of extractant. [P204], [P507], [P227], and [C272] = 1 mol L−1.

As above-mentioned, the solubility of same extractant was very close in different RE elements at the same loading ratios. And the aqueous solubility of extractants reduced gradually with the increase of loading ratios. When the loading ratios were 100%, the solubility of P204, P507, P227, and C272 were down to 0.0022 mol L−1, 0.0168 mol L−1, 0.0020 mol L−1, and 0.0071 mol L−1, respectively. The above results showed that the aqueous solubility of extractant was related to the coordination with REs. In regard to the equilibria of extraction reaction, it is often considered in terms of the following steps.25,44 (a) Partition of the extractant HL between the organic and aqueous phases:

HL(o) ⇄ HL(a)

Figure 6. Effects of different acidities in rare earths feed on the solubility of extractants. [P204], [P507], [P227], and [C272] = 1 mol L−1; the loading ratios is 30%.

solubility of the same extractant was very close for different RE elements at the same acidity, and the solubility of extractants declined gradually with the increase of aqueous acidity. In the solvent extraction process, the acid solution is used in RE extraction and the stripping unit. The effects of higher acid concentration on a stripping solution were discussed in what follows. 4. The Solubility of Organophosphorus Acid Extractants in Stripping Solution. To define the solubility of extractant in the stripping solutions, we used the 6 mol L−1 hydrochloric acid to strip the oil phase whose RE loading ratio was 60%. In addition, we set up a control group whose oil phase is fresh to investigate the effect of RE on the solubility of extractant in higher acid solution. The solubility of extractants in stripping solutions and pure hydrochloric acid are listed in Table 2. As observed, the solubility of P204 and P227 was higher in the stripping solution than in the pure acid solutions. This may occur because trace amounts of extractants were decomposed by the metal−salt in the stripping solution.29

(1)

(b) Dissociation of HL in the aqueous phase: HL(a) ⇄ H+(a) + L−(a)

(2)

(c) Aqueous phase reaction of L−1 and the RE ions Mn+: Mn +(a) + n L−(a) ⇄ MLn(a)

(3)

(d) Partition of the MLn complex between the organic and aqueous phases: MLn(a) + mHL(i) ⇄ MLn(HL)m(o)

(4)

(e) With a combination of eq 1−4, the overall reaction of solvent extraction of rare earth ions (Mn+) with extractant (HL) can be shown as

Table 2. Solubility of Extractants in Stripping Solutions and Pure Hydrochloric Acid

Mn +(a) + (n + m)HL(o) ⇄ MLn(HL)m(o) + nH+(a)

concentration of phosphorus (mmol L−1)

(5)

where HL and L− represent the undissociated dialkylphosphoric acid and its anion, and (a) and (o) are aqueous and organic phases, respectively. D

aqueous phase

P204

P507

P227

C272

stripping solution pure acid solution

0.98 0.40

0.49 1.78

0.41 0.04

0.12 0.65


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Industrial & Engineering Chemistry Research However, the solubility of P507 and C272 was lower in the stripping solution than in the pure acid solutions. Because the solubility of P507 and C272 are relatively large in the stripping solutions, so the salt-out effect plays a primary role on their solubility. RE plays a role as electrolytes and is solvated by water. So the number of free water molecules decrease, thereby reducing the solubility of P507 and C272. Hence the effect of electrolyte on the solubility of extractant has to be considered systematically. 5. Effect of Electrolytes on the Solubility of Organophosphorus Acid Extractants. To confirm the effect of electrolyte on the solubility of the extractants, the adjustment of ionic strength in the aqueous phase was used by NH4Cl, NH4NO3, and (NH4)2SO4. The relationship of the solubility with a series of electrolyte concentrations is plotted in Figure 7.

For 1:2 electrolytes: 1 = 0.176[M 2X]−1/2 nm κ

Table 3 lists the values of κ calculated according to eq 7 and 8 at various concentrations of electrolytes. As can be seen, the Table 3. Debye Length (κ−1) for Various Electrolytes in Water κ−1/nm

The solubility of the four extractants showed the same downtrend with the increase of electrolyte concentration. However, different types of electrolytes had different influences on the solubility. The values in (NH4)2SO4 fell lower than NH4Cl and NH4NO3. It can be explained that the electrolytes can change the electrical potential surrounding the interface which falls off exponentially with the thickness of electrical double layer.46 The characteristic length (or thickness), a distance between two plates of the capacitor model, is commonly referred to as the Debye length (κ−1). For a monovalent electrolyte (z = 1) at 25 °C (298 K), the Debye length for aqueous solutions is ⎛ε ε k T ⎞ ⎜ r 0 B2 ⎟ ⎝ 2NAe I ⎠

electrolyte concn (M)

1:1

1:2

0.03 0.07 0.1 0.5 1

1.76 1.15 0.96 0.43 0.3

1.02 0.66 0.56 0.25 0.18

double-layer thickness was cut rapidly as the electrolytic concentration increased. The thickness of the electrical double layer is inversely proportional to the electrolytic concentration and to the square of the valence of the ions. The 1:2 electrolytes have more effect on the thickness than 1:1 electrolytes. Therefore, the influence of (NH4)2SO4 is greater than NH4Cl or NH4NO3 on inhibiting the extractants distributed in the aqueous phase. As suggested above, the presence of electrolyte can change interface stability by affecting the thickness of the electrical double layer, thereby influencing the aqueous partition of extractants. Not only that, it is efficient on preventing the loss of extractant by adding inorganic electrolytes to initial aqueous phase. The result was consistent with the fact that the loss of organophosphorus extractants could be inhibited by electrolyte.31 Therefore, the relationship between electrolyte and the aqueous partition of extractants should be determined in RE extraction. 6. The Relationship between Partition of Organophosphorus Acid Extractants and Electrolytes. On the basis of the above studies, we used the Pitzer equation to correlate the activity coefficient data of P227, P204, P507, and C272 in electrolytes solution in RE extraction. To keep the extractants activity constant in the organic phase, the loading ratios of REs were 30% in this part. For the neutral uncharged solutes, the Pitzer equation47 was reduced to

Figure 7. Effect of electrolyte on the solubility of extractants: The letters A, B, C, and D represent P507, C272, P204, and P227 respectively. The numbers 1, 2, and 3 represent NH4Cl, NH4NO3, and (NH4)2SO4, respectively. [P204], [P507], [P227], and [C272] = 1 mol L−1.

1 = κ

(8)

−1

ln γi = 2 ∑ λij(I )mj + 3 ∑ ∑ μijk j

j

k

∑ mjmk j

(9)

where λij is the effect of short-range forces between species i and j (depending on ionic strength), μijk is the interaction of three solute species i, j, and k, and mi is the molality of electrolytes. The eq 9 could be applied to organophosphorus acid extractants in electrolyte solution. In this paper, the subscripts HL, H, EC, and EA represent organophosphorus acid extractant, H+, the cation of electrolytes and the anion of electrolytes, respectively. ms and mH represent the molality of electrolytes and acid. Considering that mH is far smaller than ms, μijk is far smaller than λij, the activity coefficient of extractant can be expanded and simplified as follow.

(6)

where I is the ionic strength of the electrolyte, and here the unit should be mol/m3, ε0 is the permittivity of free space, εr is the dielectric constant, kB is the Boltzmann constant, T is the absolute temperature in Kelvin, NA is the Avogadro number, and e is the elementary charge. At room temperature (25 °C), one can consider in water for 1:1 electrolytes the relation 1 = 0.304[MX]−1/2 nm (7) κ

ln γHL = A + Bms + Cms 2

(10)

For 1:1 electrolytes, E


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Industrial & Engineering Chemistry Research A = 2λHL,HmH ,

Table 4. Regressed Parameters Correlated by Pitzer Equationa

B = 2(λHL,EC + λHL,EA ),

C = 6μHL,EC,EA

extractants

A

P204 P507 P227 C272

0.4 0.073 1.205 0.634

P204 P507 P227 C272

0.428 0.085 1.286 0.555

P204 P507 P227 C272

0.409 0.176 1.263 0.586

For 1:2 electrolytes, A = 2λHL,HmH ,

B = 2(2λHL,EC + λHL,EA ),

C = 12μHL,EC,EA

When reaching to the extraction equilibrium, the chemical potential of extractants is equal between two phases. Besides, because the aqueous partition of extractant is relatively minor, the extractants activity can be considered as constants in the oil phase. If we choose the state of extractant dissolved in water as 0 reference state (γHL(aq) = 1), the activity coefficient of extractants (γHL) in the electrolyte solution could be written as 0 γHL = mHL /mHL

(11) a

where m0HL represents the solubility of extractant in pure water, mHL represent the solubility of extractants in the electrolytes solution. It should be noted that the reference state of extractant’s activity coefficient in eq 11 is different with that in the Pitzer equation, whose reference state is the state of extractant infinitely diluted in water. The conversion relation between two reference states is as follows: * + RT ln mHL γ = μ Δ + RT ln mHL γ μHL HL HL HL(∞) ln γHL − ln γHL(∞) =

ln

Δ * μHL − μHL

RT

0 mHL = ln γHL = ln γHL(∞) + D mHL

=D

R2

C

NH4Cl 0.024 0.023 0.049 0.026 NH4NO3 0.008 0.018 0.017 0.0125 (NH4)2SO4 0.015 0.012 0.027 0.160

10−04 10−04 10−04 10−04

1 0.999 0.999 0.999

6.89 × 10−05 −8.12 × 10−05 3.34 × 10−05 −4.85 × 10−06

1 1 0.999 0.994

−2.90 × 10−05 −9.88 × 10−06 −8.79 × 10−05 5.25 × 10−05

1 1 0.998 0.996

−1.47 −1.41 −4.41 −1.98

× × × ×

The pitzer equation is expressed as ln γHL = (A + D) + Bms + Cms2.

significant influence on the aqueous partition of extractants and the formation of the interfacial polymer. 7. Aqueous Phase Partition of [A336][P507]. The BifILEs [A336][P507] had high selectivity and low acid−base consumption compared with acidic extractants.20 Its environmental impact should be assessed for the sake of industrial application. The stability of extractant is an important factor in RE extraction. Research showed that the ILs would gradually drain because of repeated exposure with high acidity.48 Moreover, the different aqueous pH can adjust the partitioning of ILs because of structural variations.49 Both [A336][P507] and the saponified P507 were used to destroy hydrogen bonds of P507 dimers in order to improve the efficiency of extraction. Therefore, we compared the solubility of [A336][P507] and the saponified P507 in the aqueous phase with the acidity ranging from 0 to 6 mol L−1. The results were shown in Figure 9. The solubility of saponified P507 and [A336][P507] were both decreased with an increase of acid concentration. However, the solubility of [A336][P507] was much smaller than the saponified P507. On the basis of the Pearson acid base concept (HSAB), the hard solute is freely soluble in hard solvent and vice versa.50 For [A336][P507], the [A336]+ is soft acid which has large ionic radius and strong polarity. But, for the saponified P507, NH4+ is a hard acid with small ionic radius and weak polarity. Owing to the hard base of water, the saponified P507 is readily soluble in aqueous phase, commonly known as similarity and intermiscibility. In addition, the solubility of [A336][P507] in rare earth feeds was very small, the effect of loading ratios was unremarkable, and the dissociation of [A336][P507] did not occur in stripping solutions. Bif-ILEs [A336][P507] is an environment-friendly extractant which has a significant advantage of lower loss.

(12)

(13)

(14)

Therefore, eq 10 can be rewritten as ln γHL = (A + D) + Bms + Cms 2

B

(15)

We converted the experimentally measured solubility of extractants to the activity coefficient. The results were shown in Figure 7. Equation 15 was used to fit regress the activity coefficient. The regressed parameters and root mean square (RMS) deviation were listed in Table 4. As the RMS deviation of regression presents, the pitzer equation fitted well with the activity coefficient of extractants in different concentrations of electrolyte solutions. The regressed parameter C was much smaller than B and (A+D) so that the fit results in Figure 8 approximated the straight line. Parameter C represented the interaction of four solute species. It had little effect on short-range forces between two species. The shortrange forces are affected by ionic strength because the ion− neutral or neutral−neutral interactions are small and ignorable.47 When the electrolytic concentration increased, the activity coefficient of extractants increased and the aqueous partition of extractants decreased. Therefore, the formation rate of MLn decreased in the aqueous phase. The amount of MLn was reduced in the interface, as a result, the aggregates and polymers of the RE complex also cut down or even disappear. Therefore, we can adjust the formation rate of the complex by adding a certain amount of electrolytes. Besides, the Pitzer equation is an excellent fit with the experimental data, and the regressed parameters reflected that the electrolytes had a

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CONCLUSIONS In this study, we adopted a quantitative assessment to explore factors influencing the aqueous partition of organophosphorus acid extractants (P204, P507, P227, and C272) and Bif-ILs ([A336][P507]). For organophosphorus acid extractants, we investigated that the acidities of the aqueous phase had a strong effect on the solubility of extractants by inhibiting the dissociation of extractants. A comparison with extractants of F


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Figure 8. Relationship between activity coefficient of extarctants and the molality of electrolytes (correlated by Pitzer equation) (a) NH4Cl, (b) NH4NO3, (c) (NH4)2SO4.

Science Foundation of China (Grant 51174184), and the Key Research Program of the Chinese Academy of Science (Grant KGZD-EW-201-1).

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Figure 9. Effects of aqueous acidity on the solubility of extractants. SP507 represents the saponified P507. P507 and [A336][P507] = 1 mol L−1.

the same alkyl chain showed that the solubility was closely related to the molecular volume of extractant. In particular, the aqueous solubility of extractants will decrease after coordination with REs. Besides, the solubility of organophosphorus acid extractants in stripping solution is marginal. Otherwise, the increase of electrolyte concentration will cut down the aqueous solubility of extractants. The effect of electrolyte on aqueous partition of extractants was expounded by electrical double layer theory. In addition, the Pitzer equation was used to fit the activity coefficient of the extractant in the RE extraction. Because the aqueous partition of organophosphorus acid extractants inevitably occurss in the extraction, the solubility of Bif-ILEs [A336][P507] was explored. It was found that the solubility of [A336][P507] was slight in acid solution and rare earth feed, and the [A336][P507] did not dissociate in stripping solutions either. Moreover, comparing the saponified P507 with [A336][P507], it is found that the aqueous partition of extractant was greatly affected by the cations of extractants.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-431-8526-2646. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the National Basic Research Program of China (Grant 2012CBA01202), the Natural G


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Aqueous Partition Mechanism of Organophosphorus Extractants in

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